Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system for internal combustion engines having intake values with variable valve timing.
Internal combustion engines are conventionally known, which are provided with a variable timing system for varying valve timing of intake valves, i.e. valve opening period and/or valve lift, in dependence on the engine rotational speed.
Such variable valve timing systems are generally adapted to change the valve timing between high-speed valve timing (high-speed V/T) suitable for engine operation in a high engine rotational speed region and low-speed valve timing (low-speed V/T) suitable for engine operation in a low engine rotational speed region, to thereby enhance the trapping efficiency (combustion efficiency) and hence obtain higher engine output.
Further, the variable valve timing systems are also adapted to vary the valve timing in response to operating conditions of the engine including the engine rotational speed and load on the engine so as to always ensure the best combustion efficiency during operation of the engine.
In internal combustion engines with such valve timing systems, in changing from low-speed V/T to high-speed V/T, the engine output torque is reduced immediately before the valve timing is changed from low-speed V/T to high-speed V/T so that the changeover to high-speed V/T takes place with the engine output torque reduced, in order to avoid a large shock due to a sudden change in the output torque caused by the changeover of the valve timing (torque shock).
This will be further explained with reference to FIG. 1 showing output torque characteristics assumed at high-speed V/T and low-speed V/T. In the figure, the abscissa represents the engine rotational speed NE, and the ordinate the output torque PSE.
It will be learned from FIG. 1 that different output torque characteristics are assumed between high-speed V/T and low-speed V/T, which results in a torque shock, e.g. when the engine shifts from a so-called "lean burn" operation with low-speed V/T selected in which the air-fuel ratio of a mixture supplied to the engine is controlled to a leaner value than a stoichiometric air-fuel ratio, to an operation with high-speed V/T selected in which the air-fuel ratio is controlled to the stoichiometric air-fuel ratio or a richer value than the latter. To overcome this disadvantage, conventionally it is so controlled that in changing from low-speed V/T to high-speed V/T, the ignition timing of the engine is retarded to a point where the maximum engine output torque is obtained with low-speed V/T selected to reduce the output torque to be obtained with high-speed V/T selected, as indicated by the arrow a, then change the valve timing from low-speed V/T to high-speed V/T in a hatched region indicated by x, and thereafter gradually return the output torque toward a value to be obtained with high-speed V/T selected, to thereby mitigate the torque shock.
However, according to the prior art described above, although in changeover to high-speed V/T, actually the engine is required to produce high output torque, the output torque is temporarily dropped for changing the valve timing, resulting in insufficient output torque. It is therefore desirable that changeover of the valve timing should be effected without a drop in the engine output torque.